WebGL renderer

[aduros]

This should be straightforward, but only worth doing once WebGL becomes more available on mobile. Consider adding GLSL support to HxSL and reusing the shaders in flambe.platform.shader.

[aduros]

This is about 80% done! Build with -D flambe_enable_webgl to enable. Things still to be done:

• DONE: Texture.readPixels and writePixels.
• DONE: Texture.graphics.
• DONE: Scissor testing.
• DONE: A couple Graphics transform methods.

Performance is a bit better than the canvas renderer on desktop browsers, except for Chrome (Linux) where it's way better. No difference on the Android Chrome beta. I need to do more testing, until there's a clear performance win to using WebGL, I'm probably going to pause work on this for now.

I would happily accept patches for the remaining things though :)

[aduros]

WebGL is a lot faster than canvas on Android Chrome stable!

More TODOs:

• DONE: Wire up contextlost events to System.hasGPU.
• Compressed texture support.